1. Technical Field of the Invention
The present invention relates to integrated circuits, and more particularly to buried-gate semiconductor devices, that is to say those of which at least a portion of the gate region is placed below the channel region.
2. Description of Related Art
Among the semiconductor devices with so-called buried gates are distinguished devices with a double planar gate, that it so say in which the gate region comprises two gates, one of which is situated above the channel region and the other of which is situated below the channel region, and the so-called GAA (Gate All Around) devices in which the gate region is formed from a single part and surrounds the channel region.
Buried-gate devices, whether or not they are double-gate, are particularly advantageous for channel lengths less than 50 nanometers because they make it possible to eliminate the so-called short-channel effects while making it possible to obtain a current intensity that is doubled or tripled with respect to a conventional transistor. It is recalled here that a short channel has a very short distance (length) between the source and the drain and that the “short-channel” effect results in a reduction of the threshold voltage of the transistor, which can, at the extreme limit, result in obtaining a transistor that is very difficult to control and can finish up in the “piercing” mode.
The known buried-gate devices comprise a gate made from polysilicon. However, with the reduction of the dimensions for forthcoming semiconductor devices, a gate made of polysilicon causes the appearance of many limitations such as polydepletion, an incompatibility with dielectrics having a high dielectric constant and a gate resistivity that is too high.
One solution is notably the use of a metal gate, even if the adjustment of the threshold voltage of the device can necessitate a metal whose work function is variable. One example of metal gate devices are so called TOSI (Totally Silicided) devices which are made on the basis of a polysilicon gate above which is deposited, at the end of the manufacturing process, a metal which diffuses in the polysilicon during an annealing process. Another example of metal gate devices are devices whose polysilicon gate is etched at the end of the process, in order to deposit a metal gate in that place.
In the case of buried-gate devices, when the gate material is a metal silicide, the metal is deposited on the lateral walls of the polysilicon forming the gate region or regions. However, a poor formation of the metal silicide is obtained, notably because of the limited diffusion length of the metal in the polysilicon.
When the gate material is a metal, it is deposited under the channel regions, at the end of the transistor production process, and it must therefore be possible to deposit that material in cavities. The number of metals that can be considered is limited and the production of the device remains difficult.